1. Field of the Invention
This invention is related to a method and apparatus for decoding borehole telemetry signals transmitted by means of either a magnetic or electric dipole antenna. More particularly, this invention relates to a method and apparatus for synchronizing a receiver time base to that of the transmitter so as to enable the decoding of phase shift keyed (PSK) signals.
2. Description of Prior Art
The problem of decoding borehole telemetry signals is unusual in that the bandwidth is a large fraction of the carrier frequency, the bit rate is very low, and the signal to noise ratio is poor at the limits of range. Phase shift keying is well known, and it has long been known that the optimal means, from a signal to noise point of view, to transmit binary data over a noisy channel is to utilize 180-degree phase shifts. Phase shift is defined with respect to a constant frequency carrier. The transmitted data consists of a string of binary bits with which a time period (window) for each bit is associated. FIG. 1 is illustrative of phase shift keyed modulation. Shown is a phase shift keyed waveform, a carrier and the binary bits being transmitted. As shown, time is divided into windows, one for each bit. The sign of the waveform changes sign if the bit changes. A “1” corresponds to the case where the signal has the same phase as the carrier, while a “0” is transmitted by sending the signal with a phase 180 degrees away from that of the carrier. Correct demodulation of the phase shift keyed signal requires that the carrier and windows be known.
One known approach to the decoding of phase shift keyed signals is based upon a phase locked loop (PLL). Use of a phase locked loop in phase shift keyed decoding is complicated by the fact that the phase is reversed when the bits change. Thus, the signal must be multiplied by the demodulated bit so that it always has the same sign as the carrier before being fed to the phase locked loop. This is shown in FIG. 4. Using the known decoder shown in FIG. 4, the error signal is mixed with the oscillator output with 90-degree phase shift added. When the circuit is locked onto a signal, the error signal is very small and reflects small deviations of the oscillator from a perfect phase match. This error signal is filtered and fed into the voltage controlled oscillator (VCO) in order to maintain the lock. A second channel with the carrier not phase shifted is used to obtain the bit values. The binary bit stream can be obtained by means of a comparator to determine the sign of the signal. This method has the obvious problem that the bits are needed to demodulate the signal and to maintain lock, but the bits cannot be obtained before lock is attained. Such a method works well for tracking a signal once the phase locked loop is locked onto it; however, circuits of this kind are poor at acquiring lock and can be unlocked by noise.
Another known means for determining the carrier of a 180-degree modulated signal is to first square the signal, which removes the phase and coding information and leaves the second harmonic of the carrier. This can be locked onto by means of a simple phase locked loop or other conventional means. In principle, this latter approach can work well, but in practice, it is often complicated. The squaring operation is non-linear and noise from every part of the input signal appears at the carrier frequency. To obtain reasonable performance, the signal must be carefully filtered prior to squaring and the narrow band carrier must still be extracted from the squared signals and the windows determined. In the practical case, frequency distortion of the signal will cause the squared signal to broaden, as the phase information will no longer be totally suppressed. The carrier signals determined by this means tend to have more noise than that determined by a phase locked loop circuit which is locked onto the signal. As taught by U.S. Pat. No. 5,912,930, one approach is to first acquire lock by means of the second harmonic, or similar means independent of the bit stream, and then switch over to a phase locked loop-based scheme to maintain lock.